A lattice gas model is used to describe the adsorption of benzene in silicalite I. Using the complete equivalence of the lattice gas with an Ising model and the known exact as well as mean-field results of the latter, we study the thermodynamics of the adsorption and establish criteria on the adsorption energies and site interactions for the existence of a two-phase region. It is shown that a phase change occurs when zigzag paths connecting interacting adsorption sites become energetically favorable. Our model also leads to a sharp rise of the adsorption heat at the transition, albeit at a level less enhanced than experimentally observed.
